Fighting between Banunu and Batende communities in Mai-Ndombe province in Democratic Republic of the Congo caused killings, UN says. Meanwhile, the African Union is set to discuss the violence and allegations of election fraud, in Ethiopia meeting.
At least 900 people are believed to have been killed over three days in December in ethnic violence in northwestern Democratic Republic of the Congo or DRC, the United Nations said on Wednesday, and it warned that the toll could be higher still.
This doubles an estimate provided on Monday by a local priest and a civil society activist who said that at least 400 people had died in bloodshed that led the government to cancel voting there in last month's presidential election.
The fighting between the Banunu and Batende communities in Mai-Ndombe province was some of the worst in the area in years and highlighted the precarious state of inter-ethnic relations even in the central African country's more peaceful regions.
"I have to emphasise that 890 is the number of people we know were actually buried," UN human rights spokeswoman Ravina Shamdasani said after the release of a statement on the attacks.
"But there are reports that many others may have been killed and their bodies may have been dumped in the Congo River or they may have been burned to death," she said.
Communal fighting and widespread pillaging around the town of Yumbi led to an estimated 16,000 people seeking refuge by crossing the Congo River into the Republic of Congo, according to the UN rights office statement.
It said 465 houses and buildings, including schools, a health centre, market and office of the national electoral commission, had been burned or pillaged.
The violence broke out over a dispute linked to a tribal chief's burial, Shamdasani said.
While the bloodshed was not directly related to the end-of-year election, a local activist told Reuters news agency in December that tensions between the two ethnic groups had festered because Batende leaders were supporting the ruling coalition while Banunu leaders backed opposition candidates.
The December 30 vote was meant to lead to DRC's first democratic handover of power since independence from Belgium in 1960. But hope for a new era after 18 years of chaotic rule by President Joseph Kabila has faded in a welter of controversy.
The second-place finisher, Martin Fayulu, has contested the result.
African Union to debate disputed elections
Meanwhile, continental leaders will gather at the African Union this week to discuss the disputed election in DRC, a spokeswoman for the body said on Wednesday.
The DRC election commission last Thursday declared opposition leader Felix Tshisekedi the winner of the December 30 vote with 38.57 percent of the tally against chief rival Fayulu's 34.8 percent.
Fayulu has appealed the result, saying it was an "electoral coup" forged in backroom dealings between Tshisekedi and outgoing President Kabila, who has been in power since 2001.
The Thursday meeting at AU headquarters in Ethiopia was called by Rwandan President Paul Kagame, chairman of the body until next month, spokeswoman Ebba Kalondo said.
"The initiative is part of the African-led efforts to assist the DRC political stakeholders and people to successfully conclude the electoral process," she told AFP news agency.
South African President Cyril Ramaphosa announced his attendance on Twitter, though it remained unclear which other leaders would join him.
Struggling with instability
Vast and unstable, DRC has never had a peaceful transition of power since gaining independence from Belgium in 1960.
It became a battlefield for two regional wars in 1996-97 and 1998-2003, and the last two presidential elections, in 2006 and 2011, were marked by bloody clashes.
Now the country's top court has eight days from when Fayulu's appeal was lodged on January 11 to render a verdict.